1. Field of the Invention
The present invention relates generally to the field of lithography. More specifically, the present invention relates to the mounting of a projection system of a lithographic projection apparatus.
2. Description of Related Art
The term “patterning device” as here employed should be broadly interpreted as referring to devices that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning devices include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. When using extreme ultraviolet (EUV) radiation, only reflective masks are suitable, for reasons well known to a person skilled in the art. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic devices. In both of the situations described here above, the patterning devices can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning devices as here above set forth.        
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning devices may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step and repeat apparatus. In an alternative apparatus—commonly referred to as a step and scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti parallel to this direction; since, in general, the projection system will have a magnification factor M (generally<1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion implantation (doping), metallization, oxidation, chemo mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are, then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0 07 067250 4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
The projection system as described above usually comprises one or more, for instance six, optical elements, such as lenses and/or mirrors. The optical elements direct the projection beam through the projection system and project it onto the target portion. In case the radiation system supplies a projection beam of EUV-radiation, mirrors should be used instead of lenses, in order to direct the projection beam, since lenses are not translucent to EUV-radiation.
When an extreme ultraviolet projection beam is used for projecting relatively small patterns, the demands for the projection system concerning the accuracy are rather high. For instance, a mirror, which is positioned with a tilting error of 1 nm, can result in a projection error of approximately 4 nm on the wafer.
A projection system for projecting an extreme ultraviolet projection beam comprises, for instance, 6 mirrors. Usually, one of the mirrors has a fixed spatial orientation, while the other five are mounted on e.g. Lorentz-force actuated mounts. These mounts can preferably adjust the orientation of the mirrors in 6 degrees of freedom (6-DoF-mounts) using 6 Lorentz-force motors per mirror. The projection system further comprises sensors for measuring the spatial orientation of the mirrors.
The projection system is mounted on a reference, or metrology frame, by means of, for example, compliant mounting devices. These mounting devices present low-pass characteristics, having a cut-off or Eigen frequency of, for example, about 30 Hz. This is done to stabilize the projection beam and isolate it from vibrations and disturbances coming from the environment, such as adjacent systems. The reference frame is, in turn, mounted on a so-called base frame through a very soft mount, in conventional lithographic apparatus called an ‘air mount’. These mounts have low-pass characteristics and may have an Eigen frequency of approximately 0.5 Hz. Onto the reference frame, amongst others, an interferometer wafer stage position measurement system is mounted. The mirrors in the projection system need to be positioned very accurately with respect to each other, whereas the wafer stage and reticle stage need to be positioned very accurately with respect to this set of mirrors again.
Currently, positioning the mirrors relative to each other is being done by attaching one rigidly to a frame of the projection system, hereinafter called the projection optics box or POB. The other mirrors are actively positioned relative to the POB. Positioning accuracy depends, amongst other things, on the level of disturbance of the POB. Currently, the level of disturbance of the POB is too high to rigidly attach the POB to the reference frame. Possible causes for disturbance are:                Base frame motion; the reference frame is mounted on this base frame through springs which are very soft but still finite in stiffness. So, vibration of the base frame causes some force disturbance being introduced into the reference frame. This base frame, in turn, just stands on the floor, thus being subject to floor vibrations. Also, stage acceleration forces, left over after balancing, are being exerted onto this base frame, again causing vibration;        Air mount noise. They cause noise-like force disturbance exerted onto the reference frame;        Noise from water-cooling. It is expected that cooling of the reference frame is required or desired. This will be done by water-cooling. The flow of water through the pipes and tubes also causes force noise to be exerted onto the reference frame; and        Internal dynamical modes of the reference frame or of other modules attached to the reference frame.Because current positioning systems use passive elements, like soft mounts, for suppressing vibrations in the projection system, vibrations coming from a reference, or metrology frame still affect the projection system, mounted thereon.        